tests: Gateway: parse_and_error_handle: Two tests to get started

This is Athanasius feeling out how best to add tests to this ancient code.

* Test that eddn.Gateway.parse_and_error_handle() returns the correct error
  string for:
  - Invalid JSON being passed in.
  - An outdated schema being cited.

src/tests/gateway/test_parse_and_error_handle.py

@@ -0,0 +1,28 @@
+import os
+import sys
+
+# Tests don't include the directory that `pytest` is run from on sys.path
+sys.path.append(os.getcwd())
+
+import eddn.Gateway
+
+def test_bad_json():
+    msg = "{not real json"
+    res = eddn.Gateway.parse_and_error_handle(msg.encode(encoding="utf-8"))
+    assert res.startswith("FAIL: JSON parsing: ")
+
+def test_outdated_schema():
+    msg = """
+{
+    "$schemaRef": "http://schemas.elite-markets.net/eddn/journal/1",
+    "header": {
+        "uploaderID": "outdated schema",
+        "softwareName": "pytest:Gateway.parse_and_error_handle",
+        "softwareVersion": "v0.0.1"
+    },
+    "message": {
+	}
+}
+    """
+    res = eddn.Gateway.parse_and_error_handle(msg.encode(encoding="utf-8"))
+    assert res.startswith("FAIL: Outdated Schema: The schema you have used is no longer supported. Please check for an updated version of your application.")